Thumb motion is a key outcome metric for assessing disease progression or treatment efficacy. A literature review found nearly 25 % of recent papers incorrectly described their motion measurements as those of the first carpometacarpal (CMC) joint, when in fact their technology was only capable of measuring thumb motion. The aim of this manuscript is to clarify the importance of the accurate terminology and to rigorously examine the potential error by comparing thumb motion and CMC joint motion. Computed tomography (CT) images from 46 healthy subjects were analyzed using 3D markerless bone registration techniques to compute thumb rotation (first metacarpal (MC1) relative to the radius) and CMC joint rotation (MC1 relative to trapezium). We found thumb rotation was a poor measure of CMC joint rotation. For example, at thumb rotations of 20°, the true CMC joint rotations ranged from 3° to 30°. On average, thumb rotation over predicted CMC rotation by approximately 10°, with 95 % Limits of Agreement ranging from 30° (over estimating CMC joint motion) to −11° (underestimating CMC joint motion). Importantly, the character of the data demonstrated that CMC motion cannot be predicted from thumb motion. 3D CMC joint motion can only be assessed with skeletal imaging technologies;​ goniometers and skin-based markers can, at best, only measure thumb motion. Referring to goniometer and skin marker measurements as CMC joint motion is incorrect. It is critical that investigators be precise in their reporting of thumb motion versus CMC joint motion, especially when reporting interventions for thumb pathologies.